The field of solar heating can be divided generally into two types of systems: (1) active, and (2) passive.
In an active system, there is a solar energy collector which may include radiation focusing, directing or concentrating means plus a heat transfer fluid which is pumped through the collector, a circuit for conducting the heat transfer fluid (usually a liquid), a pump, pump control means, plus a heat exchanger through which the heat transfer fluid is pumped after it has been heated by the solar collector. From this heat exchanger, the heat energy is made available for heating the desired living space. Other elements, such as heat storage units, may be included in an active system.
Such active systems suffer the disadvantages that they are relatively sophisticated or complex and expensive. Active systems are also critical in their operation, particularly when the heat transfer fluid is a liquid, since leakage or non-circulation of the liquid on a hot mid-summer day can result in overheating of the collector leading to its damage or destruction. Other critical factors result from the fact that the solar collector plus its radiation focusing, directing or concentrating means, must be installed in an exposed location where they are subject to the attrition of weather and battering of storms. Such exposed solar collectors suffer the added disadvantage that they rapidly lose heat in the winter and become cold whenever the sky is overcast during the hours from dusk to dawn, when incident solar radiation does not occur.
Moreover, many active systems require a relatively high temperature to be attained in the solar collector itself before the system can raise the heat transfer liquid up to the desired relatively high temperature, as required by the heat exchanger. Such relatively high temperature requirements in the collector, lead to the result that very little useful solar heat energy can be collected on partially overcast or intermittently cloudy days and cause rapid heat loss at night.
The present invention does not lie in the field of active solar heating systems, which are non-analogous in many aspects of this invention.
In a passive system, the building structure itself, in which is enclosed the living space, is arranged to receive and to retain much of the heat energy from the incident solar radiation during winter months. This structure is also arranged to reject, reflect and dissipate much of the heat energy from the incident solar radiation during summer months when the sun is higher in the sky and remains above the horizon for more hours during the day, for preventing overheating of the living space. Advantageously, passive systems have few moving parts, and the exterior surfaces of the building structure itself are those elements which are exposed to the weather. In other words, exterior collector elements are not employed, and the exterior surfaces of the building structure itself tend to have much greater durability against weathering and storm damage than exposed collector elements.
Furthermore, passive systems have the great advantage that they are inherently relatively low temperature systems. The temperature levels to be attained from incident solar radiation need only be a few degrees above the temperature level desired to be provided in the living space itself, for example 68.degree. F. Therefore, passive solar systems can capture much useful solar energy on partially overcast or intermittently cloudy days and can even capture a significant amount of useful heat from diffuse sky radiation on moderately overcast winter days even when the ambient temperature is low. Under such conditions, active solar heating systems are essentially ineffective.
The direct components of a passive solar heating system are (1) an aperture facing within 45.degree. of the true south, constructed in such a manner that its surface when glazed will be at an angle approximately perpendicular to the sun's rays at winter solstice; (2) a glazing covering said aperture which maximizes the transmission of solar radiation while minimizing through its construction or by insulative additions the outward flow of heat via radiation, convection and conduction; and (3) storage means which absorbs solar radiation and re-releases this thermal energy at times of need to maintain appropriate interior space temperatures. Additional components which may be added to passive systems are fans which provide for thermo-circulation from the solar heated space to adjacent or connected spaces requiring heat.
This invention lies in the field of passive solar heating systems employing solar greenhouses. The presently preferred embodiments of the method and system of this invention, as described in this specification, include relatively few moving parts. A single fan is utilized and controlled by a differential thermostat. During winter operation, the rate of energy flow (thermal flux) from the greenhouse to the attached structure to be heated, is controlled by the fan which, in turn, is controlled by the thermal differential between the greenhouse and attached structure. Advantageously, the air is drawn by this fan over the units of thermal storage material as the air is being circulated into the attached structure. In the summer, the fan is reversed to provide for expelling heated air from the attached structure and from the greenhouse for preventing overheating. Depending on harshness of climate, two removable thermal barriers may be utilized which are open during the daytime for admitting solar radiation and are closed at nighttime for enclosing the interior of the solar greenhouse to prevent undue heat loss at night. One of these removable barriers covers the upper portion of the interior of the greenhouse and the other covers the side or sides of this region.
In view of the fact that air is circulated, this passive solar heating method and system of the present invention may be considered hybrid in nature, but in its major thrust this invention provides all of the advantages of a passive system plus many others as will be described later on.
A detailed paper, "Design Considerations, Theoretical Predictions, and Performance of an Attached Solar Greenhouse Used to Heat a Building", published in the Proceedings of the June 6-10, 1977 Annual Meeting of the American Section of the International Solar Energy Society by two of the current inventors (Taff and Holdridge) outlines early work on the hybrid greenhouse systems which preceded the current invention. In addition, in an earlier paper entitled "Solar Sustenance Project" by William F. Yanda (also one of the current inventors), published by the International Solar Energy Society in the Proceedings of the July 28-Aug. 1, 1975 International Solar Energy Congress and Exposition, the author clearly laid out the groundwork for the symbiotic thermal relationship between an attached solar greenhouse used as a passive solar heater and its benefits to an attached dwelling. These two papers reflect early work of the inventors and set the foundation for their current invention.
A recent book which describes the current state of the art in solar greenhouses is The Food and Heat Producing Solar Greenhouse (Revised and Expanded Sixth Printing) by Bill Yanda and Rick Fisher, available from John Muir Publications, Inc., P.O. Box 613, Santa Fe, N.Mex. 87501. This publication describes certain principles and thermal characteristics of solar greenhouses, including the effects of added mass within the greenhouse, such as barrels of water against the north wall (FIGS. 7,9 and 214), or water in a sunken fish tank (FIG. 167) for absorbing and storing heat, or water in barrels over the entrance (FIG. 180), or in storage tubes and containers (FIG. 197), and the effect of heat storage mass in heat storage walls or in rock beds (FIGS. 105, 110, 129, 142 and 152). This book describes the basic "greenhouse effect" (FIG. 5) and illustrates various possible geometrics for solar greenhouses and describes the effects of placement or orientation of such structures relative to the sun's rays at various times, at various latitudes. Also described are the use of movable insulation (FIGS. 37-45, inclusive), roll-up insulating curtain walls (FIGS. 191, 192, 193) and a pivoted insulator reflector curtain (FIG. 158). The final Chapter VIII reviews "The State of The Art" including reference to attached solar greenhouses. Appendix E describes "Heat Distribution in the Attached Greenhouse on a Clear Winter Day".
In the Arizona Highways issue for May, 1980 is an article entitled "Solar Energy--Where It Is and Where It's Going", plus other articles about utilization of solar energy. The main focus of the home heating discussions is on active solar heating systems, but there is some discussion of passive systems.
The technical publication SOLAR ROOM, written by two of the present inventors, and published by Garden Way Publishing of Charlotte, Vt., in 1976, describes attached solar greenhouses and free-standing solar greenhouses.
A publication Advance ZeroThermic Greenhouses shows a pre-engineered modular greenhouse using urethane insulation and connected to a home by air ducts.
The prior art solar greenhouse structures described, plus the other information in these publications provide certain advantages, but they do not anticipate nor render obvious the present invention.
In spite of all of their numerous advantages, as indicated from the above comparison between passive and active systems, our recent analyses have revealed four significant problem areas causing serious shortcomings in performance of prior art solar greenhouses. These problem areas are (1) heat storage, (2) unduly large heat loss through the glazing, (3) excessive east/west glazing, and (4) inefficient overall geometric configuration including inadequate south facing aperture.
If insufficient thermal storage mass is provided in prior art structures, then late, during cold winter nights and during the subsequent early morning hours, the stored heat energy is all consumed, allowing temperatures in the space to drop relatively freely to unattractively low levels during these late night and early morning periods of time.
On the other hand, if excess thermal mass not having latent heat properties is provided, such excess produces a resultant temperature level in the greenhouse wherein the daily fluctuations are minimized, but unfortunately such a temperature level is usually too low to provide any useful heat to the attached structure. Thus, the result is an optimized growth environment for plants in the greenhouse itself, but the greenhouse no longer serves as a useful producer of heat for the attached structure.
An important principle determining the solar heating capacity of the attached greenhouse is to keep nighttime temperatures therein no lower than a predetermined threshold, which we calculate to be 42.degree. F., while maximizing those periods when the greenhouse interior is at a temperature sufficient to provide useful heat for delivery to the attached structure, which we calculate to be at least 68.degree. F. but no greater than approximately 80.degree. F., and also while maximizing those periods when thermal energy is being stored by direct absorbtion into a latent heat material active in this range.
The objective of the present invention is to "harvest" economically significant amounts of solar energy for heating a building; thus, maximizing "harvestability" of solar radiation generated heat from a solar greenhouse is an important objective.
This above-stated predetermined threshold or lower bound (calculated to be 42.degree. F.) is selected to allow viable plant growth while minimizing the need for auxiliary heating of the interior of the greenhouse.
The above-stated upper boundary range (calculated to be 68.degree. F. to 80.degree. F.) is selected to be adequate for producing desired heating of the attached structure while at the same time minimizing heat loss through external panels and glazing of the greenhouse. (It is to be understood that as the temperature in the interior of the greenhouse increases, the differential .DELTA.F..degree. between interior and exterior temperatures directly increases, and the heat loss to the exterior approximately linearly increases with increase in .DELTA.F..degree..)
Movable insulation in many prior art solar greenhouses is often in relatively heavy panel form and in many cases is required to be installed in high or narrow locations in which the insulation is awkward to handle, thereby calling for considerable labor in set up and removal. Thus, the average homeowner may tend, from time-to-time, to avoid or postpone the deployment of such insulation. This failure to set up in early evening during winter causes rapid loss of stored heat energy and upsets the operation of the passive system, resulting in erratic temperature drops in the living space. If there are plants and vegetables growing in the greenhouse, they can be quickly damaged or killed by such rapidly falling temperatures.